High selectivity and ion-exchange character of birnessite-type sorbents make these materials promising for Sr-90 removal from radioactive wastes containing seawater. However, it is still debatable how their functional properties are related to the nature of active phases and structure and, therefore, routes to improve birnessite performance are virtually unknown. Here we study the evolution of structure, surface morphology, and redox and sorption properties of birnessite-type sorbents in sorption/regeneration cycles and after reductive treatment with hydrazine and hydrogen to enhance birnessite uptake of Sr-90 from highly mineralized solutions. The influence of various treatment procedures on physico-chemical characteristics of birnessite has been evaluated using XRD, SEM, low temperature nitrogen sorption, and temperature-programmed reduction. Temperature programmed reduction was implemented to reveal changes in birnessite's active sites distribution and morphology, which play a crucial role in sorption behavior of the material. The sorption activity towards Sr-90 has been evaluated for the birnessites under static and dynamic conditions. The original approach used for characterization of birnessite sorption materials has helped to reveal how these sorbents change under operating conditions, and provided routes to the targeted modification of these manganese-oxide materials.